A fundamental process in IC fabrication is chemical vapor deposition (CVD), which uses vapor precursors to deposit thin films on a semiconductor substrate. The reactor used for CVD processes includes a precursor delivery system, a substrate and an energy source to decompose the precursor vapor to reactive species to allow a thin film to form on the substrate (CVD process). Effective power sources are heat and plasma energy such as radio frequency (RF) power, microwave energy (MW), low frequency (10 KHz-1 MHz) power, and optical energy (e.g. a laser or ultraviolet light) which decompose the introduced precursors. Plasma energy power is below 6000W. The amount of power required in each process is determined by the process reaction and a typical power level is between 500-1000W. Also, the substrate could be biased or heated (to 100° C.-1200° C.) to promote the reaction of the decomposed atoms or molecules and to control the physical properties of the formed films.
Traditionally, precursors used in semiconductor CVD processes are gaseous. An example of a CVD process to deposit silicon dioxide (SiO2) is to use gaseous precursors such as silane gas (SiH4) and oxygen gas (O2):SiH4(gas)+O2(gas)−(heat)→SiO2(solid)+2H2(gas)
The basic requirements of a precursor are that the desired product (in this example, SiO2) is solid and that all of the other products are gases (in this example, H2) which can be exhausted away. The energy required for the reaction to take place is the thermal energy which is about 400-800° C.
To broaden the processes, more and more liquid and solid precursors have been used, especially in the area of metal-organic chemical vapor deposition (MOCVD). To perform this task, a liquid precursor is typically first turned into vapor which decomposes and reacts on the substrate. A solid precursor must often be dissolved into a solvent to form a liquid precursor. The liquid precursor then must be converted into the vapor phase before being introduced into the deposition zone. An example of a CVD process to deposit copper (Cu) uses liquid precursor vapor copper HexaFluoroACetylacetone TriMethylVinylSilane (hfac-copper-tmvs, C5HO2F6—Cu—C5H12Si):2Cu-hfac-tmvs (vapor)−(heat)→Cu (solid)+hfac-Cu-hfac (gas)+2tmvs (gas)
Another deposition technique is the atomic layer epitaxy (ALE) process. In ALE, the precursors are pulsed sequentially into the ALE process chamber. Each precursor sequentially generates a chemical surface reaction at the substrate surface to grow about an atomic layer of the material on the surface. The growth of one atomic layer in ALE is controlled by a saturating surface reaction between the substrate and each of the precursors. Sometimes a reduction sequence activated with extra energy such as heat or photon is used to re-establish the surface for a new atomic layer. The fundamental criterion of ALE is to have a minimum of two different chemical reactions at the surface with each reaction being carefully controlled to generate only one atomic layer. An example of ALE is the growth of ZnS at ˜470° C. using sequential flow of elemental zinc and sulfur as precursors as disclosed in U.S. Pat. No. 4,058,430 to Suntola et al. Another example of ALE is the growth of germanium (Ge) on a silicon substrate at ˜260-270° C. by first pulsing GeH4 vapor to generate an atomic cover layer of GeH4 and pulsing Xe lamp radiation to decompose the surface GeH4 as disclosed by Sakuraba et al, J. Cryst. Growth, 115(1-4) (1991) page 79.
The ALE process is a special case of atomic layer deposition (ALD). The focus of ALE is the deposition of epitaxial layers, which means forming perfect crystal structures. In contrast, the ALD process seeks to deposit one layer at a time with the focus on forming film uniformity, and not on creating single crystal structures.
The major drawbacks of CVD and ALD processes are the high temperatures needed for the chemical reactions and the limited number of available precursors. CVD and ALD processes always start with an extensive evaluation of various potential precursors and their chemical reactions to determine see if there is a suitable process reaction.
To lower the temperature needed for the chemical reaction, and to further promote possible reactions, plasma energy can be used to excite the precursors before the reaction takes place in CVD processes. Such processes are called plasma enhanced CVD (PECVD). An energy source using radio frequency (RF) power or microwave (MV) power can be used to generate a plasma, which is a mixture of excited gaseous species, to supply energy to the precursors to promote chemical reactions.
However, there is no plasma enhanced ALD process or equipment. The main advantages of a plasma enhanced ALD would be the low temperatures required for the reactions to occur, and the increased number of precursors made available by the addition of plasma energy to excite the precursors. Furthermore, plasma treatment could modify the surface condition which also leads to a wider range of available precursors.
It would be advantageous to develop a plasma enhanced ALD system.
It would be advantageous if plasma treatment could be incorporated in an ALD process.